The distillation process requires more energy than any other single operation in the entire chemical industry. Despite this, distillation is still, in many applications, a preferred separation method. In many situations, distillation is the only means to separate two or more combined materials.
Distillation involves stripping and rectifying, both typically conducted in the same tower at different heights along the tower. In the lower portion of the tower below the feed tray, a mixture is heated causing it to boil. The generated vapors pass upwardly through the column. At the feed tray, the same mixture in liquid form is injected into the column. This liquid flows downwardly, contacting the rising vapors. The vapors preferentially strip the more volatile components of the mixture from the downwardly flowing liquid. Vapors continue to rise upwardly into the upper rectifying portion of the tower and pass completely out of the column. A portion of these vapors are condensed and injected at the top into the column at its highest point. This condensed liquid contacts the upwardly flowing vapors causing the less volatile material to condense.
The separation occurring in the stripping section is effected by the vaporization of the more volatile component. The separation in the rectifying section is caused by the condensation of the less volatile material. As such, in order to improve efficiency, it would be desirable to heat the stripping section and to cool the rectifying section. To make the separation most effective, it is desirable to actually place heat exchangers within the distillation column. At the upper rectifying sections, cooling units should be used. At the lower stripping sections, heaters should be used.
For separation purposes, this is very efficient; economically, this is very inefficient. The capital cost for providing multiple heat exchangers in very long columns makes this impractical.
A recent development has been to use split columns, separate stripping and rectifying columns. In such systems, the gas evolved from the stripping column is directed to the rectifier column, and the liquid condensate from the rectifier is returned to the upper portion of the stripping column. Such units have been further modified by using heat exchangers between the stripping column and the rectifier. Thus, the excess heat evolved in the rectifier is transferred to the stripping column to provide the additional needed heat for the stripping operation. Further unwanted heat in the rectifier is removed. Such an apparatus is disclosed in Haselden U.S. Pat. No. 4,025,398.
Again, the problem with such a system is that it is extremely complex and very capital intensive. The heat exchangers require a large deal of capital expenditure and furthermore are not completely efficient. Further, gas liquid contact within the column is decreased because of the interferences caused by the heat exchangers.